The efficiency of gas turbine engines depends significantly on the operating temperature of the various engine components with increased operating temperatures resulting in increased efficiencies. The search for increased efficiencies has led to the development of superalloys capable of withstanding increasingly higher temperatures while maintaining their structural integrity.
Nickel-base superalloys are used extensively throughout the aeroengine in turbine blade, nozzle, and shroud applications. Aeroengine designs for improved engine performance require alloys with increasingly higher temperature capability. Although shroud and nozzle applications do not require the same level of high temperature creep resistance as blade applications, they do require similar resistance to thermal mechanical failure and environmental degradation. Superalloys are used for these demanding applications because they maintain their strength at up to 90% of their melting temperature and have excellent environmental resistance.
Single crystal (SC) superalloys may be divided into “four generations” based on similarities in alloy composition and performance. A defining characteristic of so-called “first generation” SC superalloys is the absence of the alloying element rhenium (Re). For example, U.S. Pat. Nos. 5,154,884; 5,399,313; 4,582,548; and 4,209,348 each discloses superalloy compositions substantially free of Re.
A representative SC nickel-base superalloy is known in the art as AM1 having a nominal composition of: 6.0-7.0% Co, 7.0-8.0% Cr, 1.8-2.2% Mo, 5.0-6.5% W, 7.5-8.5% Ta, 5.1-5.5% Al, 1.0-1.4% Ti, 0.01 maximum % B, 0.01 maximum % Zr, and balance essentially Ni and C wherein C is specified as 0.01% (100 ppm) maximum. Mach 1 velocity cyclic oxidation Test at 2150° F. data for a Rene N4 superalloy and an AM1 superalloy are provided for comparative purposes in the accompanying Figures.
It was discovered that the addition of about 3 wt % Re to superalloy compositions provides about a 50° F. (28° C.) improvement in rupture creep capability and the accompanying fatigue benefits. Production alloys such as CMSX-4, PWA-1484 and Rene N5 all contain about 3 wt % Re. These “second-generation” alloys are disclosed, for example, in U.S. Pat. Nos. 4,719,080; 4,643,782; 6,074,602 and 6,444,057.
U.S. Pat. No. 4,719,080 provides a relationship between compositional elements called a “P-value” defined as P=−200 Cr+80 Mo−20 Mo2−250 Ti2−50 (Ti×Ta)+15 Cb+200 W−14 W2+30 Ta−1.5 Ta2+2.5 Co+1200 Al−100 Al2+100 Re+1000 Hf−2000 Hf2+700 Hf3−2000 V−500 C−15000 B−500 Zr. The patent stresses that a higher “P-value” correlates with high strength in combination with stability, heat treatability, and resistance to oxidation and corrosion. In particular, the superalloy compositions disclosed in the patent are constrained by “P-values” greater than 3360.
U.S. Pat. No. 6,074,602 is directed to nickel-base superalloys suitable for making single-crystal castings. The superalloys disclosed therein include, in weight percentages: 5-10 Cr, 5-10 Co, 0-2 Mo, 3-8 W, 3-8 Ta, 0-2 Ti, 5-7 Al, up to 6 Re, 0.08-0.2 Hf, 0.03-0.07 C, 0.003-0.006 B, 0.0-0.04 Y, the balance being nickel and incidental impurities. These superalloys exhibit increased temperature capability, based on stress rupture strength and low and high cycle fatigue properties, as compared to the first-generation nickel-base superalloys. Further, the superalloys exhibit better resistance to cyclic oxidation degradation and hot corrosion than first-generation superalloys.
U.S. Pat. Nos. 5,151,249; 5,366,695; 6,007,645 and 6,966,956 are directed to third- and fourth-generation superalloys. Generally, third-generation superalloys are characterized by inclusion of about 6 wt % Re; fourth generation superalloys include about 6 wt % Re, as well as the alloying element Ru. These superalloy compositions illustrate the value of increased Re additions in terms of mechanical performance.
First generation SC superalloys do not offer the thermal mechanical failure (TMF) resistance or the environmental resistance required in many hot section components such as turbine nozzles and shrouds. Also, first-generation SC superalloys do not offer acceptable high temperature oxidation resistance for these components.
Currently, aeroengines predominantly use second-generation type superalloys in an increasing number of hot section applications. The alloying element Re is the most potent solid solution strengthener known for this class of superalloys and therefore it has been used extensively as an alloying addition in SC and columnar-grained directionally solidified (DS) superalloys. The second-generation superalloys exhibit exceptional high temperature oxidation capability balanced with satisfactory mechanical properties.
Known superalloy compositions having lower Re content have not been able to provide the properties obtainable from second-generation superalloys. In particular, in U.S. Pat. No. 4,719,080, the data for one alloy (namely, B1) having less than 2.9% Re show properties comparable to first-generation, i.e., no Re, superalloys. Thus, in the development of superalloy compositions, the trend has been to use at least 3 wt % Re to obtain a satisfactory balance of oxidation resistance and high temperature strength.
However, the cost of the raw materials, and the global shortage of Re in particular, provides a challenge to develop superalloy compositions able to provide the demonstrated improved mechanical properties and oxidation resistance of second generation superalloys, but at low, and preferably 0% Re levels. Heretofore, second-generation properties in nickel base superalloys having less than 3 wt % Re has previously not been attained.
Accordingly, it would be desirable to provide nickel-base superalloy compositions having less than 3 wt % Re content that are able to provide single-crystal and directionally solidified articles having required high temperature characteristics.